Field-effect solar cells using crystalline silicon (c-Si) and hydrogenated amorphous silicon have been developed. In such cells, the doped contact layers (p+ or n+) in conventional p+-n or p-i-n structures are replaced with field-effect inversion/accumulation charge layers. This is beneficial for solar cells based on thin-film materials such as a-Si:H with low doping efficiency (and therefore limited separation of quasi-Fermi levels) and poor diffusion length in the doped layers. Using the inversion layer instead of the doped layer can improve (i) the open-circuit voltage (Voc) of the cells by increasing the separation of the quasi-Fermi levels, and (ii) the short-circuit current (Jsc) of the cells by eliminating light absorption in the doped layers. The solar cells described above involve using an external power supply or a dielectric fixed charge for inducing electronic charge in the contact region.
An amorphous silicon thin-film transistor (a-Si:H TFT) 20 is shown in FIG. 1. This type of transistor has a wide variety of uses, including in TFT/LCD displays and in the production of x-ray medical imagers. The transistor includes a substrate 22 adjoined by a gate metal 24 and a layer 25 of a-SiNx:H. An intrinsic amorphous hydrogenated silicon layer 26 adjoins the a-SiNx:H layer. A pair of doped, amorphous hydrogenated silicon contact regions 28, 30 (n+ a-Si:H) adjoin the intrinsic layer 26.
An amorphous silicon p-i-n solar cell 40 is shown in FIG. 2. This exemplary structure includes a substrate 42, a transparent conductive layer 44 (a TCO in the exemplary structure), a p+ amorphous hydrogenated silicon layer 46, an intrinsic amorphous hydrogenated silicon semiconductor layer 48, and a n+ a-Si:H contact region 50. The poor quality of doped contacts in amorphous materials (low doping efficiency, high defect density and relatively strong light) limit efficiency of the structure.